Synthesis of Tetrahydroquinolines via an Intramolecular Aza-Diels-Alder Reaction

Significance

Anilines have been shown to par­ticipate in dearomatizitive aza-Diels-Alder reactions with aldehydo cinnamate based dienophiles to give predominantly trans,trans-tetrahydroquino­lines in yields generally above 90%. Mild conditions using TFA at r.t. were developed although other acids [e.g., Yb(OTf)₃, tartaric acid] were successfully tested and showed negligible effects on the obtained yields and stereoselectivities. In contrast, other solvents led to reduced yields. Anilines bearing either electron-donating or -withdrawing groups give higher yields than unsubstituted anilines. The dienophiles were conveniently and efficiently synthesized in four steps starting from glycine methyl ester.